#include <stdio.h>
#include <math.h>
double f(double x,double y,double z,double d)
{double j;
 double k;
 double l;
 j=z-x;
 k=d-y;
 l=sqrt(j*j+k*k);
 return l;
}
int main()
{double a;
 double b;
 double c;
 double n;
while(scanf("%lf%lf%lf%lf",&a,&b,&c,&n))
{
    printf("%lf",f(a,b,c,n));
}
return 0;
}